1. Field of the Invention
The present invention relates to a method of wind tunnel measurement of airfoil, and more particularly relates to a method of wind tunnel measurement of airfoil, which is used for in a windmill, an aircraft, a turbine and the like.
2. Description of Related Art
As an apparatus that uses a blade, a windmill, an aircraft, a turbine and the like are known. The blade of the windmill, the aircraft or the like is high in aspect ratio (fineness ratio) (e.g., 10 to 20 or more). For this reason, in many cases, the aerodynamic design and the calculation of the aerodynamic performance of the entire blade are carried out by determining the aerodynamic performance of a two-dimensional airfoil section and integrating it in a blade width direction and then estimating a three-dimensional performance. In that case, it is necessary to carry out a wind tunnel test under a condition close to an actual use condition to get the aerodynamic characteristics of the two-dimensional airfoil section.
One of the conditions for the wind tunnel test is to secure the two-dimensional flow property. The flow of the fluid in the actual entire blade is three-dimensional. However, as mentioned above, the aspect ratio of the actual blade is high. Thus, from the viewpoint of each blade element, the flow of the fluid can be considered to be two-dimensional except the blade ends. Hence, in the wind tunnel test, it is important to remove the three-dimensional flow property of the fluid and secure its two-dimensional flow property.
As a related technique, Japanese Patent Publication No. JP-A-Heisei 9-210839 discloses a wind tunnel test apparatus for a structure. This wind tunnel test apparatus for a structure includes: a wind guiding path through which air flows; a model which is arranged in the wind guiding path and has an axis serving as a rotation center; a suspending wire suspending and holding the model in the wind guiding path; a detector which detects various forces generated in the model through the suspending wire when the model receives the flow of air; a model rotating mechanism which has a motor and a decelerator interlocking with the motor and rotates the model around the axis to change an orientation of the model with respect to a direction of the flow of air; and an actuator which is remotely operated. This wind tunnel test apparatus is characterized by containing a fixing mechanism for fixing the model in an orientation determined by the model rotating mechanism.
The inventors have now discovered the following facts.
The inventors have studied the following method as a method of a wind tunnel test. FIG. 1 is a schematic view showing the method of the wind tunnel test studied by the inventors. As shown in FIG. 1, a wind tunnel test apparatus 101 includes two walls 102, two supporting members 104 and two load cells 103. Each of the two walls 102 has a flat surface parallel to an x-direction. The two walls 102 are arranged at a predetermined distance from each other in a z-direction. The space between the walls 102 configures a wind tunnel flow path. An airfoil (airfoil model 111) under test is arranged in the wind tunnel flow path. A fluid (air)) for the wind tunnel test flows through the wind tunnel flow path. The upper supporting member 104 is arranged to penetrate through the upper wall 102 in the z-direction and not to interfere with the upper wall 102. The lower supporting member 104 is arranged to penetrate through the lower wall 102 in the z-direction and not to interfere with the lower wall 102. In the upper supporting member 104, one end is coupled with the top end of the airfoil model 111, and the other end is coupled with the upper load cell 103. In the lower supporter 104, one end is coupled with the bottom end of the airfoil model 111, and the other end is coupled with the lower load cell 103. The two load cells 103 fix the airfoil model 111 in the z-direction through the supporting members 104, respectively. The load cells 103 measure loads in the x-direction and the y-direction that are applied to the airfoil model 111 at the time of the wind tunnel test. The airfoil model 111 has the shape in which the airfoil is cut away in the two flat surfaces vertical to the longitudinal direction of the airfoil. The two supporting members 104 are connected to both ends of the airfoil model 111 that correspond to the two cutaway surfaces, respectively.
In this way, when the airfoil model 111 is placed inside the wind tunnel flow path and then the load is measured by using the load cells 103, a pair of balances or the like, it is considered that the following case occurs. That is, the airfoil model 111 is moved in the z-direction, then the airfoil model 111 is brought into contact with the surface of the wall 102, and consequently the load in the airfoil model 111 cannot be properly measured. Thus, in order to avoid the influence on the z-direction displacement of the airfoil model 111, it is required to form a gap between the airfoil model 111 and each of the two walls 102. FIG. 2 is a schematic view showing a relation between the airfoil model 111 and the wall 102. As shown in FIG. 2, a gap with an interval d1 is formed between the airfoil model 111 and the wall 102. For example, in the case of the airfoil model 111 having an airfoil chord length of 1500 mm, the interval d1 is approximately 10 mm.
However, with the existence of this gap, there is a possibility that a gap flow 110 occurs and/or an airfoil tip vortex (not shown) increases. Here, the gap flow 110 is the flow that the fluid moves from the side of a positive pressure surface to the side of a negative pressure surface in the airfoil model 111. In that case, the flow of the fluid is not two-dimensional originally scheduled but three-dimensional. This results in the loss of the lifting power of the airfoil model 111. Thus, it is difficult to obtain the aerodynamic characteristics of the two-dimensional airfoil section that is originally purposed.